Welcome to the forum.
I don't know if there is a way. If those who created the files used mathml instead of images the equations would look much better (I Just made the experiment, epub3 with mathml converted in kepub). I also noticed the problem with a sample book a lot time ago downloaded for curiosity, but luckily I don't need to read equations.
I have tried to visualize a file with a single equation rendered as an image that I created myself (for which the test is not very significant) with Koreader and Plato and the image, even if it is grainy is bigger. It sucks but it reads. You could try one of them.
https://www.mobileread.com/forums/sh...d.php?t=314220
I hope that someone else will have a better solution for You.
edit: Of course Koreader or Plato will not be able to open the books You bought from Kobo because of DRM issues but there is Apprentice Half for that. In this forum it is not allowed to say more.